Spray the grill with no-stick cooking spray or rub with pieces of fat. Prepare a fire in the charcoal grill about an hour and a half before cooking the tenderloin. The coals should be gray, not red hot. Douse lightly with water and wait a few minutes before proceeding if the coals are too hot.

Make 1/2-inch incisions all over the surface of the pork. Insert the slivers of garlic into the tenderloin pushing gently with the fingers until well studded all over. Season with salt and pepper. Cook uncovered over low heat turning frequently, until lightly browned on the outside and inside shows no further signs of pink, about 45 minutes according to the heat of the charcoal grill. A meat thermometer should read 160°F when positioned at its thickest point. Let rest 10 minutes before slicing.